Ginny,
It is important to get whatever exercise you can but listen to your body....when it says too much, quit. You probably cannot do what you could in the past but that does not mean you will not benefit from exercise. Just tailor it to what you can do and do not push yourself. A sick and already stressed body will not benefit.
I have been fit all my life and worked out for over 55 years. 6 months after my first flare (I have mctd, lupus, scleroderma and polymyositis) I lost 40 lbs of muscle and started 2006 as a quadriplegic who could not swallow. I have fought my way back and now work out 5 times a week, lift weights, do tai chi, run, play golf....but it is not the same body or fitness level as before.
When I first got back on an elliptical machine 18 months ago I could only do 20-30 seconds at minimum resistance. I literally felt like I was going to die. Now, 20 minutes at a high resistance is just part of my workouts and I have been able to run, slowly, about 1/2 mile. Not sure how far I can walk but 5 miles is no problem. Not a normal fitness level but light years better than 2 years ago.
Point is that you should do what you can but don't overdo it. As you feel better you can push a little bit but always listen to your body. Fatigue is a constant "friend" for almost all of us, even those of us who are improving.
Just keep moving even if slowly.
Bill